1. Field of the Invention
The present invention relates generally to a method of fabricating a gas sensor and the product fabricated thereby.
2. Description of Prior Art
A gas sensor is a device which will change its resistance value when gas molecules are affixed thereto. Materials for fabricating gas sensors includes solid state metallic oxides such as SnO.sub.2, ZnO, WO.sub.3, Fe.sub.2 O.sub.3, or In.sub.2 O.sub.3 etc. Elements such as Pd, Pt, Ni, Sb, or rare earth metal elements can be added to the material to increase the gas sensing sensitivity thereof. The gas sensor can be utilized for detecting the presence of gases such as alcoholic odorant, hydrogen, petrochemicals, or carbon monoxide etc.
Conventionally, there are three processes which can be utilized for the fabrication of the gas sensor--sintering, thick film, and thin film.
It is a general requirement that the gas sensor be porous and with a large gas sensing surface area. Therefore, if sintering is utilized, the process has to be carefully monitored not to excessively sinter the gas sensor to deteriorate the demanded quality. However, this will cause weakness to the strength of the bonding between the molecules of the gas sensing elements and thus the gas sensor will break easily. Furthermore, compared with the thick film and the thin film method which utilize integrated circuit techniques, the sintering method is not well adapted for automation and consumes more materials in fabrication. Consequently, the sintering method has a higher unit cost for fabricating gas sensors and thus is less suited for mass production.
As for the thick film method, since the heat treatment thereof only allows a low temperature in the range from 400.degree. to 600.degree. C. and a short period of less than one hour, the mechanical strength of the fabricated gas sensor is also weak.
As for the thin film method, the fabricated gas sensor has a high sensitivity. However, the process requires instruments for thin film coating that are extremely expensive.
In a Japanese Laid-Open Patent No. 63-233,358, a method of fabricating a gas sensor is disclosed. The method utilizes a screen printing technique for printing electrodes with a conducting alloy of platinum and tungsten onto an insular substrate, and subsequently for printing a gas sensing layer with a paste made of metallo-organic compounds and binder onto the electrodes. The printed layers are then heat treated.
In the foregoing method, the electrodes (the conducting alloy) are printed before forming the gas sensing layer. As a result, to prevent the electrodes from being oxidized, the heat treatment to the gas sensing layer has to be made only under a low temperature (400.degree. C.-600.degree. C.) and with a short period of time (less than one hour). This restriction, however, causes the bonding between the gas sensing layer and the substrate to be weak, resulting in a gas sensing layer that peels off easily from the substrate.
In addition, utilizing the screen printing technique, it is unable to fabricate the gas sensing layer to a very thin thickness. Material cost is therefore high and quality is nonetheless unsatisfactory.
Furthermore, it is generally an elaborate and tedious procedure to prepare the paste used for making the gas sensing layer. Skilled technicians are therefore demanded for the fabrication of the gas sensor.
In a technical paper entitled "Tin Oxide Gas Sensing Microsensors from Metallo-Organic Deposited (MOD) Thin Films" and published in Ceramic Engineering and Science Proceedings, pp.1095-1105, 1987 by Micheli et al, there is disclosed another method by which a gas sensor is fabricated with an integrated thin film heater formed on a suspended thin membrane. In the method, however, the heat treatment to the gas sensing element is still restricted to a temperature range from 400.degree. C. to 600.degree. C. Consequently, the bonding between the gas sensing layer and the substrate is also weak. In addition, since integrated circuit techniques are utilized in this method, high-cost equipments are required and the complicated fabricating procedures thereof require highly skilled technicians to operate. Costs are therefore very high.